The invention relates generally to detecting conditions of rotatable members and has particular application to detecting the pressure of a vehicle tire.
A number of patent specifications have addressed the problem of detecting and indicating the pressure of a vehicle tire. These documents disclose methods of using the variations in tire pressure to modify one or more parameters of a resonant circuit which is attached to the vehicle wheel. In many of those cases, resonant primary, secondary or receiving circuits are used, at least two such circuits being electrically closely coupled. For example UK No. 2 065 898, UK No. 2 069 209, U.S. Pat. No. 2,274,557, U.S. Pat. No. 3,662,335 and European application No. 0 045 401 disclose systems having closely coupled circuits. Such closely coupled circuits, or mutually tuned circuits, have the disadvantage that characteristics of the electrical signals generated by the circuits such as their bandwidth and amplitude are dependent on this coupling, which is in turn dependent on the distance between the circuits. The results is that these parameters of the electrical signals vary undesirably with variations in the distance between the circuits, which increases the difficulty of obtaining accurate and reliable measurements. French Pat. No. 2 344 007 discloses an arrangement employing an FM transmitter on the wheel.